Desirable performance of centrifugal pumps is required not only at high flow but also at low flow. When a pump is required to operate for reduced pressure head, it ends up consuming more power as the excess pressure is dissipated in the valves by closing the valves by nearly 50% or more by increasing System resistance. To avoid energy wastage during the low flow, the pumping assemblies known in the art utilize a variable frequency drive with a motor, a drive or a pulley drive to control the flow by varying the speed of pump shaft rotation. However, employment of the pulley drive requires stopping the pump for the change of speed. Further, the use of a gear box or the variable frequency drive unit increases the procurement costs of the pumping assembly.
In order to avoid using gear box, frequency drive or pulley unit, the prior art technologies have used pumps with multiple impellers housed in the multiple chambers where one of the chamber is closed for flow. Nevertheless, the impeller in the closed chamber continues to rotate and heats the fluid thereby wasting energy.
Other solutions how to avoid the energy wastage of the pumping assemblies were to furnish the pumps with disengage-able impellers in order to prevent rotation of the impeller in the closed chamber so that the heating of the medium is eliminated. A friction clutch is provided to engage and disengage one of the impellers according to the flow requirements. However, the employment of the friction clutch is rather expensive as the clutch manufacturing and maintenance is costly. Furthermore, additional costs incur due to employment of high capacity bearings or additional bearings as the higher axial force is required to keep the clutch faces together for higher transmission. Another disadvantage of pump assemblies using the friction clutch is that the clutch needs to be immersed in oil so its application is restricted to pumping oil and clear fluids.
Employment of the clutch for the engagement and disengagement of impellers during low flow results in desired energy saving. But the energy saving is less due to the slip loss in the clutch. Pumps with clutch utilization known in art can operate in parallel or in serial relationship. For pumps operating in parallel, one high flow and one low flow impellers are used which results in unstable pressure output due to mismatch of the pressure versus flow curve of the dissimilar impellers. On the other hand, in the pumps in serial relationship, the resultant flow corresponds to the smaller flow of the one of equal or unequal size impeller. Hence, the large flow capacity (in case of unequal impellers) of the other impeller is not utilized.
Thus, there is a need for a pump with multiple engage-able impellers, which overcomes the above mentioned drawbacks and provides energy-saving, low maintenance-cost pump assembly with stable performance at low flow.